The pipeline used for transport of natural gas, crude oil or the like over a great distance is desired to have improved transport efficiency. In order to improve the transport efficiency, the operating pressure of the pipeline must be increased, while the strength of the line pipe must be improved corresponding to the increase in the operating pressure.
The pipeline having an increased thickness has higher strength but the increased thickness lowers the welding work efficiency at the operation site. Furthermore, the increased thickness increases the weight of the line pipe accordingly, and therefore lowers the working efficiency at the time of constructing the pipeline. Therefore, approaches to increase the strength of the material of the line pipe itself have been taken rather than increasing the thickness. Today, line pipes having a yield strength of at least 551 MPa and a tensile strength of at least 620 MPa are commercially available, a typical example of which is X80 grade steel standardized by the American Petroleum Institute (API).
By the way, there have been pipeline constructions in progress in cold regions such as in Canada in recent years, and the line pipe used in such a cold region must have high toughness and high propagating shear fracture arrestability. The propagating shear fracture arrestability refers to the capability of arresting a crack if any from further propagating from any brittle fracture caused by a defect inevitably generated at a weld zone.
The line pipe must have good weldability in terms of welding work efficiency.
Therefore, the line pipe must have high strength, high toughness, and high propagating shear fracture arrestability.
JP 2003-328080 A, JP 2004-124167 A, and JP 2004-124168 A disclose steel pipes having high toughness, deformability and strength by the use of a steel pipe base material containing fine carbonitrides having oxide of Mg and Al enclosed therein and composite materials of oxides and sulfides. However, the composite materials of oxides and sulfides should lower the propagating shear fracture arrestability of the steel.
JP 2004-43911 A discloses a line pipe having its low temperature toughness improved by reducing the Si and Al contents in the base material. A method of producing the disclosed line pipe is not specified, and therefore there could be segregation or the crystal grains could be coarse. In such a case, the propagating shear fracture arrestability is lowered.
Another related document is JP 2002-220634 A.